I am using Thunderbird on my Windows PC with my gmail.com account, too. It is generally working fine.
I have an issue now by sending emails to a contact also with gmail.com email: they are rejected with the following error message:
host gmail-smtp-in.l.google.com [18.104.22.168]
SMTP error from remote mail server after pipelined end of data:
550-5.7.26 This mail has been blocked because the sender is unauthenticated.
550-5.7.26 Gmail requires all senders to authenticate with either SPF or DKIM.
550-5.7.26 Authentication results:
550-5.7.26 DKIM = did not pass
550-5.7.26 SPF [gmail.com] with ip: [22.214.171.124] = did not pass
550-5.7.26 To mitigate this issue, please visit Gmail's authentication guide
550 5.7.26 https://support.google.com/mail/answer/81126#authentication dk5-20020a170907940500b009b274ce51e2si1417324ejc.280 - gsmtp
Reporting-MTA: dns; vie01a-dmta-ch01.mx.upcmail.net
I am not sure who should fix this issue, me or Sunrise (*.upcmail.net)…
What do you think?
Thanks & Best Regards,